This invention relates to a process for preparing N-hydroxyalkylaminomethylphosphonic acid or salts thereof. In one aspect, this invention relates to a new and useful process for preparing N-hydroxyalkylaminomethylphosphonic acid or salts thereof from an alkanolamine, formaldehyde and a dialkyl phosphite. In another aspect, this invention relates to a process for preparing N-phosphonomethylaminocarboxylic acid or salts thereof by the catalytic oxidation of N-hydroxyalkylaminomethylphosphonic acid or salts thereof. In yet another aspect, this invention relates to a process for preparing N-phosphonomethylglycine useful as a herbicide.
N-hydroxyalkylaminomethylphosphonic acid or salts thereof are useful as a raw material in the preparation of agricultural chemicals. N-phosphonomethylaminocarboxylic acids or salts thereof are useful as agricultural chemicals. Specifically, N-phosphonomethylglycine, known also by its common name glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling a large variety of weeds and crops. It is applied to the foliage of a very broad spectrum of perennial and annual grasses and broad-leafed plants to achieve the desired control. Industrial uses include control of weeds along roadsides, waterways, transmission lines, in storage areas, and in other nonagricultural areas. Usually glyphosate is formulated into herbicidal compositions in the form of its various salts which retain the anionic form of glyphosate in solution, preferably in water.
The reaction of a primary amine with an aldehyde and a phosphite diester is disclosed in Fields, "The Synthesis of Esters of Substituted Amino Phosphonic Acids" J. Am. Chem. Soc., Vol. 74, pp 1528-31 (1952) However, the reaction product contains considerable amounts of the undesirable bis-phosphonomethylated product. Similarly, the reaction of a primary amine with formaldehyde and phosphorous acid is disclosed in Moedritzer and Irani, "The Direct Synthesis of .alpha.-Aminomethylphosphonic Acids. Mannich-Type Reactions with Orthophosphorous Acid", J. Org. Chem., Vol 31, pp. 1603-1607 (1966). As in Fields, the reaction product is predominantly the bis-phosphonomethylated product.
Barsukov et al, "Synthesis of New Complexons and Their Derivatives", Zhurnal Obshchei Khimii, Vol. 53, No. 6, pp. 1243-49 (1983) and Barsukov et al, "Synthesis of New Complexons of the Aliphatic Series and Investigation of the Mechanism of Acidic Dissociation", Zhurnal Obshchei Khimii, Vol. 55, No. 7, pp. 1594-1600 (1985) disclose the reaction of ethanolamine with paraformaldehyde and dimethyl hydrogen phosphite with a mole ratio of amine/phosphite of 1.0 and a mole ratio of formaldehyde/amine of 1.0. While these articles disclose that the product is the mono-phosphonomethylated compound, reproduction of the reaction disclosed in the experimental section resulted in no mono-phosphonomethylated product being produced, i.e. .sup.31 P-NMR of the material analyzed in the Barsukov example showed a 0% yield of N-(2-hydroxyethyl)aminomethylphosphonic acid (See Example 3 herein). The process as described in the Barsukov et al articles is, therefore, incapable of preparing N-hydroxyalkylaminomethylphosphonic acid in a commercially practicable manner.
A process for preparing N-hydroxyalkylaminomethylphosphonic acid or salts thereof which is economical, commercially viable, and can produce essentially only the mono-phosphonomethylated product is highly desirable.